In a speech and audio encoding algorithm, due to the limit of the bit rate and characteristics of the hearing sense of human beings, low frequency information is always encoded with priority, and high frequency signal is discarded. With the development of the network, the limits to bandwidth get looser, and people have higher requirements on tone quality. The tone quality of signals can be well improved by increasing signal bandwidth, that is, by restoring the high frequency signal. When no bit or only a few bits are available, the bandwidth expansion technique is always used. The bandwidth expansion technique is a technique of expanding the speech and audio signal bandwidth range to improve the signal quality. In recent years, the bandwidth expansion technique has been developed quickly, and commercially applied in some fields. The bandwidth expansion algorithm in G.729.1 and the Spectral Band Replication (SBR) in the Moving Picture Experts Group (MPEG) are two bandwidth expansion algorithms that are extensively used in speech and audio compression presently.
The high frequency signal can be classified into transient signal and non-transient signal. A transient signal refers to the signal of which the signal energy value in the time domain is subject to transient great changes. In an existing method for encoding a high frequency signal, when a high frequency signal is encoded, a transient signal and a non-transient signal can be encoded in different manners. But due to the development of the time-frequency transformation technique, the method for encoding a high frequency signal can no longer be compatible with the current time-frequency transformation technique. Therefore, the existing method for encoding a high frequency signal may degrade the performance of transient signals.
In an existing method for decoding a high frequency signal, a high frequency excitation spectrum is normalized according to the length of calculating a frequency envelope in encoder. As a result, the deviation of the characteristics of the restored high frequency signal from the characteristics of the actual high frequency signal is huge.